WebSocketServer handshake handling

[liamaharon]

Hi, thanks for the crate!

I've added a HandshakeHandler to the ws ServerConfig, allowing auth check(s) during http handshake rather than needing to wait until the connection is upgraded.

Useful for implementing low cost auth check like the netcode connect tokens.

The new config entry is optional, so no breaking changes.

Example creating a WebSocketServer with a custom HandshakeHandler:

#[derive(Resource, Default)]
struct Counter(Arc<RwLock<u32>>);

fn start(mut commands: Commands, counter: Res<Counter>) {
    let counter = counter.0.clone();
    let predicate = move |req: &Request, mut resp: Response| {
        // Use outside state
        let mut counter = counter.write().unwrap();
        info!("Call {}", counter);
        *counter = counter.wrapping_add(1);

        // Validate the Request however we want
        let auth_header = req
            .headers()
            .get("Authorization")
            .ok_or_else(|| ErrorResponse::new(Some("missing authorization header".to_string())))?;
        if auth_header != "123" {
            return Err(ErrorResponse::new(Some("unauthorized".to_string())));
        }

        // Optionally modify the response before returning
        resp.headers_mut()
            .insert("X-Something", HeaderValue::from_static("Something"));

        Ok(resp)
    };
    let handshake_handler = HandshakeHandler::from(Arc::new(predicate));

    let config = ServerConfig::builder()
        .with_bind_default(GAME_SERVER_PORT)
        .with_no_encryption()
        .with_handshake_handler(handshake_handler);

    commands.spawn_empty().queue(WebSocketServer::open(config));
}

[aecsocket]

Thanks for the PR! This is something that I've been meaning to get around to doing, you can tell by the TODO, but I never found the time to implement it. The method of passing a callback isn't ideal - if possible I would like to use the ECS directly here like running a system or a trigger - but I don't think that's really possible to achieve with how tungstenite works, so I'm happy with the callback approach.

I'm very happy with the PR overall, just would like some minor tweaks to docs and implementation.

[aecsocket]

Also please make sure that CI clippy and doctests pass

[liamaharon]

Great suggestions, thanks @aecsocket.

[aecsocket]

Thanks for the fixes! I'll merge it now and do a 0.16.1 release